Halogenated volatile organic compounds (VOCs), including chlorinated aliphatic hydrocarbons (CAHs), are the most frequently occurring type of contaminant in soil and groundwater at Superfund and other hazardous waste sites in the United States. In 1996, the U.S. Environmental Protection Agency (EPA) estimated that cleanup of these sites will cost more than $45 billion over the next several decades.
CAHs are manmade organic compounds. They typically are manufactured from naturally occurring hydrocarbon constituents (methane, ethane, and ethene) and chlorine through various processes that substitute one or more hydrogen atoms with a chlorine atom, or selectively dechlorinate chlorinated compounds to a less chlorinated state. CAHs are used in a wide variety of applications, including uses as solvents and degreasers and in the manufacturing of raw materials. CAHs include such solvents as tetrachloroethene (PCE), trichloroethene (TCE), carbon tetrachloride (CT), chloroform (CF), and methylene chloride (MC). Historical management of wastes containing CAHs has resulted in contamination of soil and groundwater, with CAHs present at many contaminated groundwater sites in the United States. TCE is the most prevalent of those contaminants. In addition, CAHs and their degradation products, including dichloroethane (DCA), dichloroethene (DCE), and vinyl chloride (VC), tend to persist in the subsurface creating a hazard to public health and the environment.
The options available for a cost-effective and reliable technology to treat chlorinated hydrocarbon contaminants such as PCE, TCE, cis-1,2-dichlorethene (cis-1,2-DCE), and VC in groundwater have in recent years moved away from traditional pump-and-treat processes, especially in cases where:                Non-aqueous phase liquids (NAPLs), micro-emulsions or high concentration adsorbed materials are present leading to high dissolved phase concentrations.        Access to groundwater is restricted by surface structures or uses.        Local restrictions forbid the implementation of other available technologies such as air sparging or natural attenuation.        Pump-and-treat technologies have been applied, but have reached asymptotic removal rates.        Contamination is extensive and concentrations are too high for risk based closure but otherwise relatively low (typically 100-7500 ppb).        The migration of dissolved CAHs across property boundaries or into adjacent surface water presents a long-term remediation requirement.        The vertical migration of free phase CAHs (DNAPL) into underlying drinking water aquifers is a concern.        
The environmental chemistry of each site in part determines the rate of biodegradation of chlorinated solvents at that site. The initial metabolism of chlorinated solvents such as chloroethenes and chloroethanes in ground water usually involves a biochemical process described as sequential reductive dechlorination. The occurrence of different types and concentrations of electron donors such as native organic matter, and electron acceptors such as oxygen and chlorinated solvents, determines to a large degree the extent to which reductive dechlorination occurs during the natural attenuation of a site.
Laboratory studies have shown that a wide variety of organic substrates will stimulate reductive dechlorination including acetate, propionate, butyrate, benzoate, glucose, lactate, methanol, and toluene. Inexpensive, complex substrates such as molasses, cheese whey, corn steep liquor, corn oil, hydrogenated cottonseed oil beads, solid food shortening, beef tallow, melted corn oil margarine, coconut oil, soybean oil, and hydrogenated soybean oil have the potential to support complete reductive dechlorination.
Reductive dechlorination only occurs in the absence of oxygen; and, the chlorinated solvent actually substitutes for oxygen in the physiology of the microorganisms carrying out the process. As a result of the use of the chlorinated solvent during this physiological process, it is at least in part dechlorinated. Remedial treatment technologies usually introduce an oxygen scavenger to the subsurface in order to ensure that this process would occur immediately.
Heterotrophic bacteria are often used to consume dissolved oxygen, thereby reducing the redox potential in the ground water. In addition, as the bacteria grow on the organic particles, they ferment carbon and release a variety of volatile fatty acids (e.g., acetic, propionic, butyric), which diffuse from the site of fermentation into the ground water plume and serve as electron donors for other bacteria, including dehalogenators and halorespiring species. An iron source usually provides substantial reactive surface area that stimulates direct chemical dechlorination and an additional drop in the redox potential of the ground water via chemical oxygen scavenging.
Bacteria generally are categorized by: 1) the means by which they derive energy, 2) the type of electron donors they require, or 3) the source of carbon that they require. Typically, bacteria that are involved in the biodegradation of CAHs in the subsurface are chemotrophs (bacteria that derive their energy from chemical redox reactions) and use organic compounds as electron donors and sources of organic carbon (organoheterotrophs). However, bacteria are classified further by the electron acceptor that they use, and therefore the type of zone that will dominate in the subsurface. A bacteria electron acceptor class causing a redox reaction generating relatively more energy, will dominate over a bacteria electron acceptor class causing a redox reaction generating relatively less energy.
Certain micro-organisms will assist in removing oxygen and nitrates from the applied systems. Halophiles are salt-loving organisms that inhabit hypersaline environments. They include mainly prokaryotic and eukaryotic microorganisms with the capacity to balance the osmotic pressure of the environment and resist the denaturing effects of salts. Among halophilic microorganisms are a variety of heterotrophic and methanogenic archaea; photosynthetic, lithotrophic, and heterotrophic bacteria; and photosynthetic and heterotrophic eukaryotes. One the other hand, methanogens, play a vital environmental role in anaerobic environments, since they remove excess hydrogen and fermentation products that have been produced by other forms of anaerobic respiration. Methanogens typically thrive in environments in which all electron acceptors other than CO2 (such as oxygen, nitrate, trivalent iron, and sulfate) have been depleted.
Based on thermodynamic considerations, reductive dechlorination will occur only after both oxygen and nitrate have been depleted from the aquifer since oxygen and nitrate are more energetically favorable electron acceptors than chlorinated solvents. Almost any substrate that can be fermented to hydrogen and acetate can be used to enhance reductive dechlorination since these materials are used by dechlorinating microorganisms. However, hydrogen is also a substrate for methanogenic bacteria that converts it to methane. By utilizing hydrogen, the methanogens compete with dechlorinating microbes.
Ultimately, the inhibition of methanogenesis will result into lower methane production, which positively affects numerous environmental aspects of major concern, and will also help dehalogenating bacteria to more effectively utilize the environmental conditions that promote reductive dechlorination or chlorinated volatile organic compounds (CVOCs) in in-situ remediation processes.
Therefore, there is a need in the art for a method of inhibiting enzyme and coenzyme systems that are responsible for producing methane during the anaerobic reductive dechlorination process.